Resistivity of core plugs is routinely measured in the laboratory. The standard method of performing this measurement is the so-called “four-electrode measurement” in which electrical current is injected along the axis of the cylindrical core plug and the voltage drop along the length is measured. FIG. 1 shows a schematic drawing of this technique. The core plug (sample) 110 is sandwiched between two current electrodes 120a and 120b causing current to flow axially along the cylinder. Two ring or point (voltage) electrodes 140a and 140b, separated by 1 inch (2.54 cm), are used to measure the voltage drop along the length of the sample. As drawn, the voltage electrodes are rings of conductive wire wrapped around the circumference of the core plug 110. An LCR meter 160 (i.e., inductance L, capacitance C, resistance R measurement device) provides current to the electrodes 120a and 120b and measures the voltage across electrodes 140a and 140b. The resistance of the sample is calculated using Ohm's law:
                    R        =                  V          I                                    (        1        )            Given the geometry (e.g., length, cross-sectional area) of the core sample 110, the resistivity of the core material can be determined from the resistance.
The four electrode measurement is the technique of choice for most applications because it is not affected by the contact impedance between the current electrodes 120a and 120b with the rock sample 110. In a variation of the technique called the “two-electrode measurement”, current electrodes 120a, 120b are also used to measure the voltage drop along the entire length of the sample. However, this method cannot account for the contact impedance inherently present between the metal electrodes 120a, 120b and solid rock end surfaces. In yet another approach called “three-electrode measurement”, the sample is energized as in FIG. 1, but the voltage is measured between one of the current electrodes 120a, 120b and one of the voltage electrodes 140a, 140b. 
In the four electrode measurement, the resistance determined from the measured voltages and currents is not the resistance of the entire core plug 110. Rather it is the resistance of the part of the rock located between the two voltage electrodes 140a, 140b (which is typically 1 inch (2.54 cm)). Again, the resistivity of the material can be determined using the determined resistance and the known geometric parameters of the sample. With this arrangement the depth of investigation of the measurement is sufficient to sample the entire cross-section of the core sample 110. However, the axial resolution of the measurement is approximately 1 inch (2.54 cm), which is often too coarse. That is, a rock inhomogeneity along the length and within the cross-section of core may be overlooked. Due to the heterogeneous nature of the rock, in some applications it is desirable to know the resistivity at higher axial resolution such as 0.1 in (0.254 cm) or 0.25 in (0.635 cm), for example. In addition, it is desirable to know the resistivity of the sample at such high resolutions along the entire length of the core sample.